This disclosure relates to solid combustible propellant compositions for a variety of propellant applications.
Combustible solid propellants are well-known for a variety of applications, including but not limited to air bag inflators, inflator cartridges for portable pneumatic tools, rocket propulsion systems, as well as propellants for a variety of ballistic launch systems. Ammonium perchlorate has been widely used as an oxidizer in composite compositions that also include a high-energy fuel and a polymer binder. Ammonium perchlorate offers a number of desired performance features such as processability, good mechanical properties, low pressure exponent, and burning rate. However, perchlorate salts can cause environmental and health problems if released into the environment. Chronic exposure to perchlorates, even in low concentrations, has been shown to cause various thyroid problems. The problems from perchlorate salt in propellants can become acute in areas with localized persistent use of propellant compositions such as at rocket launch sites or munitions test ranges.
In view of the above, there have been efforts to develop combustible solid propellant compositions that utilize oxidizers that do not contain chlorine. Ammonium nitrate has been proposed for use as an alternative oxidizer to ammonium perchlorate. However, the use of ammonium nitrate in propellant applications has been subject to certain difficulties or limitations. For example, ammonium nitrate-containing propellant compositions have been subject to one or more of the following shortcomings: low burn rates, or burn rates exhibiting a high sensitivity to pressure, as well as to phase or other changes in crystalline structure such as may be associated with volumetric expansion such as may occur during temperature cycling over the normally expected or anticipated range of storage conditions. For example, storage conditions for warehoused components or munitions can vary widely in a range from −40° C. to about 110° C. Changes of form or structure of the ammonium nitrate crystalline structure may result in physical degradation of the solid structure or composite of the propellant composition. In particular, ammonium nitrate is known to undergo temperature-dependent changes through five phase changes, i.e., from Phase I through Phase V, with an especially significant volume change of ammonium nitrate associated with the reversible Phase IV to Phase III transition. Furthermore, such changes, even when relatively minute, can strongly influence the physical properties of a corresponding combustible solid propellant and, in turn, adversely affect the burn rate of the combustible solid propellant, to the point of even causing a catastrophic failure during ignition.
It has been found that the phase change-induced degradation of cast, extruded or pelletized ammonium nitrate-containing compositions can be mitigated if the humidity is kept extremely low. However, maintaining such low humidity level is often impractical for most manufacturing situations, so various forms of phase-stabilized ammonium nitrate compositions have been developed. In particular, ammonium nitrate has typically been phase-stabilized by admixture and/or reaction with minor amounts of additional chemical species. For example, U.S. Pat. No. 5,071,630 teaches stabilization with zinc oxide (ZnO), U.S. Pat. No. 5,641,938 teaches stabilization with potassium nitrate (KNO3), and U.S. Pat. No. 5,063,036 teaches stabilization with cupric oxide (CuO). U.S. Pat. No. 6,059,906 teaches stabilization with a molecular sieve age stabilizing agent and a strengthening agent. However, many prior art compositions utilizing alternative oxidizers to ammonium perchlorate suffer from poor burn rate or from a less than optimal combination of various factors such as density, caloric output, specific impulse, and volumetric impulse.